1. Field of the Invention
The present invention relates to a method for electroerosive machining of workpieces by pulses of changing current polarity and to an impulse or pulse generator for carrying out of the process.
2. Description of the Related Art
Electroerosive machining, as is known, is based on the discharge of a spark which takes place between a machining electrode and a workpiece electrode--hereinafter simply referred to electrode and workpiece. The electrode and the workpiece are immersed in an insulating liquid, the dielectric. Generally, the energy necessary for the spark and maintaining of the discharge is supplied by a power pulse generator. The direction of current flow or polarity of the (machining) pulse acting on the workpiece and the electrode depends predominantly on the desired erosion results, as well as on the material of the electrode and of the workpiece, the dielectric used, etc. Thus, for determining the polarity, the removal behavior on the workpiece and on the electrode as a function of the duration in time of the pulse--hereinafter referred as "pulse duration" is used for each work process.
For traditional sinker erosion by means of current pulses of constant direction of flow, the workpiece is frequently connected as cathode and the electrode as anode so that, taking into account the resolidification effect on the electrode, the undesired removal on the electrode (electrode wear) is kept as small as possible. Also, the electrode wear in the case of wire eroding plays a subordinate role, because the wire electrode is continuously replaced. In traditional wire erosion with current pulses of constant current direction, maximum removal on the workpiece is primarily desired. In this case, the workpiece is connected to the anode and the electrode to the cathode of the pulse generator.
From the disclosure of Swiss 536 166 a process for electroerosive machining of workpieces by current pulses of alternating direction of current is also known. By actuating switch means which are arranged in the feed branch of the machining unit, the direction of the current between the workpiece and the electrode is reversed during the duration of a current pulse.
It is essential for all the above erosion processes that an interpulse period (a pause between the pulses) is necessary between successive (machining) pulses in order to deionize the discharge channel which is developed between the workpiece and the electrode and in this way avoid degeneration of the erosion process. In this connection, the current or the voltage at the machining unit is set to zero--and therefore no difference in potential is present between the workpiece and the electrode during the interpulse period. The interpulse period must continue in the known processes, for at least such a time that the conductance of the discharge channel is reduced to such an extent that, via the deionization, a new discharge can take place at a new place. Interpulse periods which are too short lead to an undesired concentration of discharges at the same place and possibly to a degeneration of the erosion process.
The interpulse period between successive pulses represents, however, an undesired loss of time which has an unfavorable effect on the economy of known processes for the electroerosive machining of workpieces.
The invention is directed at remedying this situation.